This invention relates generally to devices and methods for embedding machine-readable digital data as self-clocking glyph shape codes, and, more particularly, to devices and methods for producing glyph shape codes such that the embedded digital data is unobtrusive.
The functional utility of plain paper and other types of hardcopy documents can be enhanced significantly if the human readable information that they normally convey is supplemented by adding appropriate machine readable digital data to them. Scanners can be employed for recovering this machine readable data, so the data can be employed for various purposes during the electronic processing of such documents and their human readable contents by electronic document processing systems, such as electronic copiers, text and graphic image processing systems, facsimile systems, electronic mail systems, electronic file systems, and document and character recognition equipment.
As is known, machine readable digital data can be recorded by writing two dimensional marks on a recording medium in accordance with a pattern which encodes the data either by the presence or absence of marks at a sequence of spatial locations or by the presence or absence of mark related transitions at such locations. The bar-like codes which others have proposed for recording digital data on paper utilize that type of encoding. See U.S. Pat. No. 4,692,603 titled “Optical Reader for Printed Bit-Encoded Data and Method of Reading Same,” U.S. Pat. No. 4,728,783 and U.S. Pat. No. 4,754,127 on “Method and Apparatus for Transforming Digitally Encoded Data into Printed Data Strips,” and U.S. Pat. No. 4,782,221 on “Printed Data Strip Including Bit-Encoded Information and Scanner Contrast.” Another interesting approach is to encode machine-readable digital data in the shapes of the marks of “glyphs” that are written on the recordings medium. Such shape codes are disclosed in U.S. Pat. No. 5,091,966 to Bloomberg et al., the disclosure of which is incorporated herein by reference.
Glyph shape codes have the advantage that they can be designed to have a relatively uniform appearance. For instance, a simple glyph shape code suitably is composed of small slash-like marks that are tilted to the right or left at, say, ±45 degrees for encoding 1's and 0's, respectively. However, in some situations the more or less uniformly gray appearance of such a code may be aesthetically objectionable, and may cause the tone of the image to change upon tilting of the glyph codes.
It is possible to form images themselves utilizing the glyph marks of a data block as the cells making up the image. This is done by the known method of halftone rendering of the glyph marks, for example as described in U.S. Pat. No. 5,315,098 (Tow) and U.S. Pat. No. 5,710,636 (Curry), both incorporated herein by reference in their entireties. In forming such an image, the gray level, or grayscale, values of the glyph marks is varied, for example by making the glyph symbols (for example “/” and “\”) thicker/darker or thinner/lighter as needed. The overall image formed contains the machine-readable embedded data therein, but again the individual glyph marks are not obtrusive to the unaided human eye.
However, a regular glyph block, especially in a glyph tone image, shows an apparent artifact if four neighboring cells form an empty diamond shape. As the size of a diamond is twice as large as the average spatial frequency of the whole block, it becomes visually noticeable in the image.